1. Field of the Invention
The present invention relates to a dielectric transmission line attenuator and a dielectric transmission line terminator, which are used in a millimeter-wave band or the like, and relates to a wireless communication device using the same.
2. Description of the Related Art
A millimeter-wave integrated circuit using a nonradiative dielectric guide (NRD-guide) was shown in IEICE Trans. (C-I), Vol.J73-C-I, No.3, pp.87-94 (March 1990).
In the NRD guide, a dielectric strip is disposed between two parallel conductive plates. The dielectric strip serves as a propagation region for electromagnetic waves, and spaces defined between the two parallel conductive plates at both sides of the dielectric strip serve as cutoff regions for the electromagnetic waves. As a terminator for such an NRD-guide, a resistor film for absorbing the electromagnetic waves is provided at the dielectric strip.
FIG. 12 shows the construction of the dielectric terminator (the two conductive plates which hold the dielectric strip are not shown in FIG. 12). The dielectric strip is divided into upper and lower dielectric strip parts. These two dielectric strip parts hold a resistor sheet and a dielectric sheet therebetween. As shown in FIG. 12, parts of the resistor sheet and the dielectric sheet are tapered. In these parts, impedance transformation of the NRD guide is performed. Furthermore, the energy of electromagnetic waves traveling in an LSM01 mode via the NRD-guide is absorbed in the resistor sheet. Accordingly, when the electromagnetic waves travel from the direction indicated by arrow A in FIG. 12, they are terminated at the terminator and are hardly reflected in the opposite direction.
Since impedance transformation is performed at the tapered resistor sheet in the conventional dielectric waveguide terminator shown in FIG. 12, the length of the tapered resistor sheet is required to be long in order to obtain sufficiently low reflection characteristics. Therefore, the overall length of the terminator becomes long as well. Such a dielectric waveguide terminator may be used, for example, at a predetermined port of a circulator in order to form an isolator, or at a predetermined port of a coupler in order to form a directional coupler. However, since the overall length of the terminator becomes long, a dielectric transmission line module using the isolator or the directional coupler inevitably becomes large. For miniaturization, it is effective to form a bend in the NRD guide so that, for example, even if the terminator is long, it can be located in a predetermined position. However, since mode transformation between LSM modes and LSE modes occurs at the bend, losses increase.
A dielectric transmission line attenuator can be constructed by providing the resistor film in the dielectric strip in the middle of the NRD guide. However, in order to sufficiently prevent the electromagnetic waves from being reflected at the resistor film, in the same manner as in the above dielectric transmission line terminator, the resistor film pattern must be formed to have a long, tapered shape. Accordingly, the dielectric transmission line attenuator encounters the same problem as the above dielectric transmission line terminator.
Accordingly, it is an object of the present invention to provide a dielectric transmission line attenuator and a dielectric transmission line terminator, in which overall miniaturization is achieved by shortening the length of the dielectric transmission line in the direction of propagation of electromagnetic waves via the dielectric transmission line, and to provide a wireless communication device using them.
To this end, according to a first aspect of the present invention, there is provided a dielectric transmission line attenuator used with a dielectric transmission line provided with two conductive plates substantially parallel to each other and a dielectric strip held between the two conductive plates. The dielectric transmission line attenuator includes a resistor film pattern provided along a surface where two parts of the dielectric strip are divided and substantially parallel to the two conductive plates. In the dielectric transmission line attenuator, the resistor film pattern and the two conductive plates form a transmission line, and the dielectric transmission line and the transmission line are coupled.
In the dielectric transmission line attenuator, the transmission line may be a TEM mode resonator having both ends open, having one end open and the other end short-circuited, or having both ends short-circuited. This construction enables attenuation characteristics of the transmission line to be increased in proximity to the resonant frequency of the transmission line. A sufficient amount of the attenuation can be obtained regardless of the small size of the attenuator.
In the dielectric transmission line attenuator, alternatively, a plurality of resistor film patterns are provided, and the distance between two neighboring coupling positions between each of the resistor film patterns and the dielectric strip is an odd multiple of a quarter wavelength. Because of this construction, resonators using a plurality of transmission lines function as band elimination filters, thus ensuring a considerable amount of attenuation over a predetermined bandwidth.
In the dielectric transmission line attenuator, the dielectric constant of a substrate having the resistor film pattern formed thereon may be greater than the dielectric constant of the dielectric strip. Accordingly, by the wavelength reduction effect in the substrate, the length of the transmission line can be decreased. Therefore, the attenuator can be miniaturized.
According to a second aspect of the present invention, a dielectric transmission line terminator includes a dielectric transmission line attenuator according to the first aspect of the present invention provided in proximity to one end of the dielectric strip.
According to a third aspect of the present invention, a wireless communication device includes one of a dielectric transmission line attenuator according to the first aspect of the present invention and a dielectric transmission line terminator according to the second aspect of the present invention. Because of this construction, a wireless communication device, such as a millimeter-wave radar module, using a dielectric transmission line as a transmission line can be easily miniaturized.